The present invention relates to an apparatus and method for fabricating conductive structures on substrates and, more particularly, to a method for fabricating conductive structures on substrate materials that are not compatible with processes used to fabricate conductive structures.
Multi-chip modules are presently in use in high performance high density systems. These multi-chip modules include a substrate or base material upon which a thin film circuit is deposited. This thin film circuit provides electrical interconnection for components that are electrically connected to the thin film circuit. These components that are mounted to the thin film circuit are frequently very large scale integrated circuits (VLSI).
The thin film technology used in the manufacture of these multi-chip modules provides relatively short interconnection distances and low interconnect capacitance between integrated components, which enhances the system performance. Thin film technology frequently makes use of multiple layers of thin film conducting material. These thin film conducting layers are separated by a dielectric material such as polyimide. Each layer of conducting material is defined using known processes such as a photolithographic process. Thin film processing is generally described in the article entitled "Multi-Chip Modules for High Performance Military Electronics," from Electrecon '91 Proceedings, sponsored by the Electronics Manufacturing Productivity Facility, Indianapolis, Ind., Oct. 22 and 23, 1991, and incorporated herein by reference.
The multi-chip module includes a heat sink for providing mechanical strength to the module, and for providing thermal conductivity between the base material and a chassis in which the heat sink is mounted. An adhesive layer is applied to the base material opposite the thin film circuit for bonding the base material to the heat sink. The heat sink is then mounted within a chassis that is cooled by some form of cooling, such as conduction, convection, or some combination of both. Heat generated by the integrated circuits is transferred through the base material, adhesive layer, heat sink and then to the chassis, thereby cooling the integrated circuits.
The thermal conductivity of the path between the integrated circuits and the chassis tends to be limited by both the base material and the adhesive layer. Adhesives generally have very low thermal conductivity, typically less than five watts per meter per degree Kelvin (W/M K.degree.). Base materials that are compatible with chemicals used in thin film processing typically have relatively low thermal conductivities which range between 20 and 170 watts per meter per degree Kelvin.
There is an ever present need for improved thermal conductivity between the integrated circuits and the chassis. Improved thermal conductivity tends to allow the integrated circuits to operate at faster speeds thereby improving the performance of the system. In addition, improving the thermal conductivity allows either more integrated circuits to be placed on a given size module or allows the module size to be reduced thereby reducing the system size. Finally, improved thermal conductivity tends to improve the reliability and life of the system.
These base materials should in addition to having good thermal conductivity should also have mechanical properties that are suitable for the base material. These mechanical properties include a temperature coefficient of expansion that is compatible with the temperature coefficient of expansion of the materials in the thin film circuit. In addition, the base material should have a high Young's constant or modulus of elasticity so that the substrate will not warp or bow as the polyimide layers are cured and shrink during the thin film circuit processing. Finally, the base material must be resistant to the chemicals used in the thin film processing. These chemicals include KOH, NaOH, H.sub.2 SO.sub.4 and various developers.